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					        Special Senses
Chapter 15
             Anatomy of an Eyeball

• Accessory structures
• 3 tunics (layers)
   – Fibrous (cornea & sclera)
   – Vascular (choroid)
   – Sensory (retina)
• Segments
   – Anterior divided into chambers
   – Posterior
   – Filled with humors (fluid)
• Lens
    Accessory Eye Structures
• Eye muscles
   – Rectus as named; oblique opposite
     and lateral
   – Diplopia: muscle weakness/alcohol
   – Strabismus: uncontrolled rotation
• Eyebrows
• Eyelids
   – Blink to spread secretions
   – Eyelashes trigger blinking
• Conjunctiva
   – Mucus prevents drying out
   – Conjunctivitis
• Lacrimal apparatus
   – Tears clean, protect, and moisten
   – Excess secretions
       • Emotional tears unique to humans
       • Stuffy/runny nose when cry
       • Watery eyes with cold
               Fibrous Tunic
• Avascular CT
• Sclera (white of the eye)
   – Protects and shapes
   – Muscle attachment
   – Continuous with dura mater
• Cornea (transparent)
   –   Outer stratified squamous, why?
   –   Inner simple squamous maintain clarity
   –   Innervated
   –   Transplants not rejected between people
                         Vascular Tunic
• Choroid
   – Vascularized to supply nutrients
   – Melanocytes to absorb light
• Ciliary body
   – Smooth muscle ring  ciliary muscles control lens shape
   – Ciliary processes secrete aqueous humor
   – Suspensory ligaments hold lens in place
• Iris
   – Colored portion of ciliary body
         • Brown pigment only (varies)
         • Less scatters light = blues/greens/grays (babies)
   – Encircles the pupil (2 smooth muscle layers)
                     Sensory Tunic
• Pigmented layer (outer)
   – Prevents light scattering
   – Phagocytize damaged photoreceptors
• Neural layer (inner)
   – Photoreceptors, bipolar cells, ganglion cell
      • Rods and cones
      • Blind spot (optic disc) filled
      • Macula lutea and fovea centralis
          – Rapid eye movement for rapid scene changes
   – Vascular supply from choroid and central vein/artery
      • Opthalmologist examines
• Retinal detachment when layers separate
   – Vitreous humor seeps in
   – Photoreceptors lose nutrients = blindness
                          Humors
• Anterior segment with aqueous humor
   –   Similar to CSF
   –   Continual development
   –   Nutrients & O2 to lens, cornea, & retina
   –   Blocked drainage = up pressure = glaucoma
• Posterior segment with vitreous humor
   – Transmits light, support lens, & intraocular
     pressure
   – Unchanged from embryonic development
 The Functioning Eye
• Light enters the pupil,
  regulated by the iris
• Passes through a convex lens
   – Avascular
   – Lens fibers added through life
      • Cataracts = clouding of lens due to loss of nutrients
• Lens is shaped by the ciliary body to focus light
  on the retina (accommodation)
   – Refraction of light converges to a focal point
   – Real image forms upside down and reversed
                    Visual Pathway
• Visual field
   – Overlap to provide depth perception = 3D
     vision
• Ganglion cells
• Optic nerve
• Optic chiasm
   – Nasal and temporal visual field
• Optic tract
• Thalamus
   – LGN
• Primary visual cortex
   – Conscious perception of images
               Olfactory Receptors
• Ciliated bipolar cells
   – Located in olfactory epithelium
     (pseudostratified ciliated)
   – Mucus captures and dissolves odorants
• Pass through cribriform plates
• Synapse in olfactory bulbs
• Odorant detection
   – Humans can distinguish 10,000 odors
   – Some is pain (ammonia, chili, methanol)
   – Combinations of different
     odorant/receptor binding
   – Replaceable, but responsiveness
     declines with age
          Olfactory Neural Pathway
• Olfactory receptors synapse with
  mitral cells
   – Contained in glomeruli
   – Receptor type specific
   – Refines smell
• Mitral cells signal via olfactory
  tracts
• 2 pathways
   – Olfactory cortex
   – Hypothalamus, limbic system =
     emotional connection
       Gustation
• Taste buds detect molecules in solution
   – About 10,000
• Four familiar and 1 other found in papillae
   – Sweet: organic substances
       • Alcohol, sugar, amino acids
   – Sour : acids, H+ in solution
   – Salty: inorganic salts
   – Bitter: alkaloids
       • Aspirin, nicotine, caffeine
   – Umami: glutamate & aspartate
       • Meats, cheeses, and protein-rich foods (MSG)
• Each receptor responsive to a particular type of substance
   – Often mixes
   – Many ‘tastes’ (80%) are really smell (head colds)
                           Papillae
• Fungiform
   – Mushroom shaped
   – Tops of, all over tongue
• Foliate
   – Fold in side walls
• Circumvallate
   – Largest, fewest, back of
     tongue
• Filiform
   – Hair like projections all over tongue
   – Do not have taste buds
   – Roughness
        Gustatory Neural Pathway
• Cranial nerves (VII and IX) carry
  sensations to medulla

• Relay through the thalamus into
  primary gustatory cortex

• Pathway initiates digestive
  process too
                    Regions of the Ear
• Outer ear
   – Pinna, external auditory canal, and tympanic
     membrane (separates)
• Middle ear
   – Pharyngotympanic tube equalizes pressure
     b/w middle ear and atmosphere (‘pop’)
   – Function of tympanic membrane
   – Ossicles (malleus, incus, & stapes) amplify signal
• Inner ear
   – Membranous labyrinths w/i bony labryinth
       • Cochlea houses the hearing organ
       • Vestibule report on changes of head position
           – Saccule and utricle for gravity and acceleration
           – Semicircular canals for rotation of head
                       The Cochlea
• Scala vestibuli
   – Perilymph: like CSF
   – Oval window
• Scala Tympani
   – Perilymph
   – Round window
• Scala media (Cochlear duct)
   – Endolymph: K+ rich intracellular
     fluid
   – Organ of Corti
       – Contains hair cells embedded in a basilar membrane
   – Vestibular membrane
   – Tectorial membrane bends cells as basilar membrane moves
      • Signal to auditory nerve
         Frequency and Amplitude
• Sounds detected as changes in AP’s
   – Pitch depends on frequency
      • High pitch = higher frequency
   – Basilar membrane responsive to
     certain frequencies
      • 20 to 20,000 Hz; 1500 – 4000 most
        sensitive
   – Loudness depends on amplitude
      • Louder sounds = higher amplitude
• Vigorous vibrations in cochlea =
  more bending = more AP’s
• Hair cells easily damaged due to
  prolonged exposure to certain
  frequencies
            Physiology of Hearing
• Pinna collects sound waves
   – Travel down auditory canal to tympanic membrane
   – Moves ossicles with vibrations
• Stapes pushes on oval window, in and out
   – Creates fluid pressure waves in scala vestibuli perilymph
• Pressure waves deform scala tympani to push round
  window in and out
   – Pressure changes move endolymph
   – Highest frequency at base (oval window), lowest at apex
• Pressure changes in endolymph, from perilymph changes,
  moves the basilar membrane
• Hair cells on Organ of Corti bend as they move against the
  tectorial membrane
   – Generates nerve impulses that leave via the cochlear nerve
                     Auditory Pathway
• AP signals from cochlea to medulla
    – Cochlear nuclei

• Some fibers cross to olives (collection of
  nuclei in the medulla) , all ascend into
  MGN(medial geniculate nucleus) in the
  thalamus
    – Pass through inferior colliculi (reflex
      area)
    – Interactions with superior colliculi to
      turn toward sound

• Synapse in primary auditory cortex

• Localization utilizes relative intensity
  and timing
                                                http://openlearn.open.ac.uk/file.php/3373/SD329_1_027i.jpg
               Dynamic Equilibrium
• Maintain body position
  after initiation of mov’t
• Within semicircular
  canals
   – Rotation within 1 of 3
     planes
   – Endolymph moves
     opposite direction of
     mov’t
   – Reverse to signal stop
      • Dizzy feeling
                Static Equilibrium
• Linear changes only
  – E.g. elevator changes or car
    acceleration/deceleration
• Vestibule
  – Saccule: vertical, hairs
    horizontal
  – Utricle: horizontal, hairs
    vertical
     • Maculae overlaid by otoliths
     • Mov’t displaces in opposite
       direction
              Motion Sickness
• Results from conflict between eyes and
  equilibrium sensors in the inner ear
  – Feeling motion, but not seeing it (inside structure)
  – One system is hallucinating, implying toxins in
    system = vomiting
• Dramamine inhibits input from equilibrium
  sensors
• Astronauts learn to control

				
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posted:11/11/2011
language:English
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